This invention relates to new crosslinking agents for epoxy resin compositions, more particularly to phosphorus element-containing compounds useful as crosslinking agents for epoxy resin compositions to yield non-halogenated, ignition resistant phosphorus element-containing epoxy resin formulations. Even more particularly, the new phosphorus element-containing crosslinking agents of the present invention are based on isomeric mixtures of tris(2-hydroxyphenyl)phosphine oxides having the following general chemical structure of Formula I: 
wherein R may be independently a hydrogen or a C1-C10 alkyl group such as methyl, ethyl, propyl, butyl, etc. The flame retardant epoxy resin formulations of this invention are advantageously used for making laminates for printed wiring boards and composite materials.
It is known to make electrical laminates and other composites from a fibrous reinforcement and an epoxy-containing matrix resin. Examples of suitable, processes usually contain the following steps:
(1) an epoxy-containing formulation is applied to or impregnated into a substrate by rolling, dipping, spraying, other known techniques and/or combinations thereof. The substrate is typically a woven or nonwoven fiber mat containing, for instance, glass fibers or paper.
(2) The impregnated substrate is xe2x80x9cB-stagedxe2x80x9d by heating at a temperature sufficient to draw off solvent in the epoxy formulation and optionally to partially cure the epoxy formulation, so that the impregnated substrate can be handled easily. The xe2x80x9cB-stagingxe2x80x9d step is usually carried out at a temperature of from 90xc2x0 C. to 210xc2x0 C. and for a time of from 1 minute to 15 minutes. The impregnated substrate that results from B-staging is called a xe2x80x9cprepregxe2x80x9d. The temperature is most commonly 100xc2x0 C. for composites and 130xc2x0 C. to 200xc2x0 C. for electrical laminates.
(3) One or more sheets of prepreg are stacked or laid up in alternating layers with one or more sheets of a conductive material, such as copper foil, if an electrical laminate is desired.
(4) The laid-up sheets are pressed at high temperature and pressure for a time sufficient to cure the resin and form a laminate. The temperature of this lamination step is usually between 100xc2x0 C. and 230xc2x0 C., and is most often between 165xc2x0 C. and 190xc2x0 C. The lamination step may also be carried out in two or more stages, such as a first stage between 100xc2x0 C. and 150xc2x0 C. and a second stage at between 165xc2x0 C. and 190xc2x0 C. The pressure is usually between 50 N/cm2 and 500 N/cm2. The lamination step is usually carried out for a time of from 1 to 200 minutes, and most often for 45 to 90 minutes. The lamination step may optionally be carried out at higher temperatures for shorter times (such as in continuous lamination processes) or for longer times at lower temperatures (such as in low energy press processes).
(5) Optionally, the resulting laminate, for example a copper-clad laminate, may be post-treated by heating for a time at high temperature and ambient pressure. The temperature of post-treatment is usually between 120xc2x0 C. and 250xc2x0 C. The post-treatment time usually is between 30 minutes and 12 hours.
It is conventional in the preparation of epoxy-containing laminates to incorporate into the epoxy resin composition various additives to improve the flame-retardancy of the resulting laminate. Many types of flame retardant additives have been suggested, but the additives which are most widely used commercially are halogen-containing additives, such as tetrabromo-diphenylolpropane, or epoxy resins prepared by reacting diglycidyl ether of bisphenol-A with tetrabromodiphenylolpropane. Typically, in order to reach the desired fire retardancy level (V-0 in the standard xe2x80x9cUnderwriters Laboratoryxe2x80x9d test method UL 94), levels of such bromine-containing flame retardant additives are required which provide a bromine content of from 10 wt % to 25 wt % based on the total polymer weight in the product.
Although halogen-containing fire-retardant additives such as tetrabromodiphenylolpropane are effective, they are considered by some to be undesirable from an environmental standpoint, and in recent years there has been increasing interest in the formulation of halogen-free epoxy resins, which are able to meet the fire retardancy requirements.
Proposals have been made to use phosphorus-based flame retardants instead of halogenated fire retardants in epoxy resin formulations as described in, for example, EP A 0384939, EP A 0384940, EP A 0408990, DE A 4308184, DE A 4308185, DE A 4308187, WO A 96/07685, and WO A 96/07686. In these formulations a phosphorus flame retardant is pre-reacted with an epoxy resin to form a di- or multifunctional epoxy resin which is then cured with an amino cross-linker such as dicyandiamide, sulfanilamide, or some other nitrogen element-containing cross-linker to form the network.
There are some commercially available phosphorus-based fire retardant additives which may be useful for replacing halogen-containing fire-retardant additives. For example, Amgard(trademark) V19 and Antiblaze(trademark) 1045 (previously Amgard(trademark) P45) supplied by Albright and Wilson Ltd, United Kingdom, are commercially available phosphonic acid ester fire retardant materials. These phosphonic acid esters, may be solids or liquids.
Alkyl and aryl substituted phosphonic acid esters are compatible with epoxy resins. In particular lower (i.e., C1-C4) alkyl esters of phosphonic acid are of value because they contain a high proportion of phosphorus, and are thus able to impart good fire retardant properties upon resins in which they are incorporated. However, the phosphonic acid esters are not satisfactory as a substitute for halogenated flame retardants in epoxy resins for the production of electrical laminates, because the use of phosphonic acid esters in amounts sufficient to provide the necessary flame retardancy increases the tendency of the resulting cured epoxy resin to absorb moisture. The moisture absorbency of the cured laminate board is very significant, because laminates containing high levels of moisture tend to blister and fail, when introduced to a bath of liquid solder at temperatures around 260xc2x0 C., a typical step in the manufacture of printed wiring boards.
Another system, which utilizes a phosphorus-based flame retardant, is described in EP A 0754728. EP A 0754728 describes the production of a flame retardant epoxy resin system by blending an epoxy resin with a cyclic phosphonate as a flame retardant and incorporating the cyclic phosphonate into the cured resin. The epoxide resin and phosphonate mixture is crosslinked with a polyamine such as triethylamine, tetra amine, polyamido amines, multi basic acids or their anhydrides for example phthalic anhydride or hexahydrophthalic anhydride. EP A 0,754,728 indicates that large quantities, such as in excess of 18 wt %, of the phosphorus additive are needed in order for the resin system to meet UL 94 V-0.
WO 99/00451 discloses flame retardant epoxy resin compositions utilizing phosphonic acid esters. WO 99/00451 discloses the reaction of a phosphonic acid ester with an epoxy resin in the presence of a catalyst and a nitrogen-containing crosslinking agent. The crosslinking agent has an amine functionality of at least 2 and is preferably dicyandiamide. The epoxy resins described in WO 99/00451 have improved flame retardant properties at low levels of phosphonic acid ester flame retardant. However, there is still a need in the industry for a flame retardant epoxy resin with improved Tg and flame retardant properties.
As aforementioned, halogen-containing phenol compounds such as tetrabromobisphenol-A (TBBA) are well known materials used in epoxy resins, specifically for use in the manufacture of FR-4 laminates for printed circuit boards. Halogen-containing compounds, specifically bromine-containing materials have the disadvantages of corrosive acidic components, e.g. HBr, released at high temperatures. It would be desirous to provide a non-halogenated material as a fire retardant additive to replace halogen-containing phenol compounds such as TBBA.
The prior art describes the use of certain phosphorus element-containing compounds as crosslinking or curing agents for use with epoxy resins as a way to introduce a phosphorus element into epoxy resin systems. For example, U.S. Pat. Nos. 4,973,631 and 5,086,156 describe the use of difunctional phosphine oxide crosslinkers such as a triphenyl phosphine oxide epoxy curing agent and a trihydrocarbyl phosphine oxide epoxy curing agent having the following chemical structural formula: 
wherein Ar is an phenyl ring and the X moieties have an active hydrogen and include an amine, hydroxy, carboxy, anhydride and thiol moieties.
JP 61,134,395 [86,134,395] teaches the use of tris(3-hydroxyphenyl)phosphine oxide having the following chemical structural formula: 
as a crosslinking agent for epoxy resins.
The above-mentioned prior art compositions are not easily prepared and require exotic preparation procedures. It would be advantageous to provide a compound that can be derived from practical, industrial scale raw materials; and thus, would offer an economic advantage over the prior art processes.
The prior art describes the use of phosphates [OP(OR)3] and other compounds with Pxe2x80x94Oxe2x80x94C units in epoxy resin formulations. Such resins do not provide a satisfactory resistance to water uptake. It would be desirous to provide compounds such as triarylphosphine oxides which do not contain Pxe2x80x94Oxe2x80x94C bonds, and therefore, do not suffer the disadvantages of the prior art compounds having Pxe2x80x94C units. It would be desirous to provide phosphorus compounds with Pxe2x80x94C linkages having superior resistance to water uptake when compared to phosphorus compounds with Pxe2x80x94Oxe2x80x94C units.
Phosphorus-containing compounds have been used heretofore in a variety of epoxy-based polymer systems including for example FR-4 laminates for printed circuit boards, container coatings, as well as high-molecular weight epoxy-based thermoplastics. A driving force for this work has been the search for non-halogenated alternatives to brominated epoxy resins. For a variety of reasons, brominated flame retardants have been deemed environmentally unsound and the use of brominated flame retardants in flame retardant epoxy resins applications has been continually threatened by legislation. Thus, there is a need to address this issue.
Commercially available non-brominated flame retardants have been used as replacements for tetrabromobisphenol-A (TBBA) in FR-4 laminates for printed circuit boards. Although most of these materials are found to meet the requirements for ignition and heat resistance, excessive moisture uptake in the board is found to be the main problem with their use. The presence of excess moisture cause the laminate boards to fail blister resistance testing. Phosphorus-containing species (phosphates, phosphonates, and phosphoramides) that contained Pxe2x80x94O and Pxe2x80x94N bonds, are believed to be the cause of excessive moisture uptake due to their polarity. Thus, it is desired to provide materials that contain Pxe2x80x94C bonds instead of Pxe2x80x94O or Pxe2x80x94N bonds in FR-4 laminate applications. It would be advantageous if such materials would reduce the amount of water absorbed by the finished laminate board. It is desired to provide compounds, which improve resistance to moisture uptake in the laminate boards, while maintaining the requirements of excellent Tg and ignition resistance.
The present invention is directed to epoxy resins which meet the desirable standards of fire retardancy without the need for halogen-containing flame retardants, or at least employing significantly lower levels of such halogen-containing flame retardants than is conventional in the art.
One aspect of the present invention is directed to new phosphorus element-containing crosslinking agents for epoxy resin compositions, more particularly crosslinking agents based on an isomeric mixture of two or more isomers of tris(2-hydroxyphenyl)phosphine oxides having the general chemical structure of Formula I as follows: 
wherein R is independently a hydrogen or a C1-C10 alkyl group such as methyl, ethyl, propyl, butyl, etc.
Another aspect of the present invention is directed to curable epoxy resin compositions containing a curable epoxy resin and, as a phosphorus element-containing crosslinking agent, an effective curing amount of a tris(2-hydroxyphenyl)phosphine oxide having the general chemical structure of Formula I as follows: 
wherein R is independently a hydrogen or a C1-C10 alkyl group such as methyl, ethyl, propyl, butyl, etc.
Yet another aspect of the present invention is directed to preparing a non-halogenated, ignition resistant epoxy resin formulation useful for making laminates for printed wiring boards and composite materials using the above phosphorus element-containing crosslinking agent of Formula I.
Hydroxyphenylphosphine oxides, such as those having the above general Formula I, which contain less hydrophilic phosphorus-phenyl units, are advantageously used for the synthesis of epoxy-based thermosets. Compounds of Formula I can be incorporated into epoxy-based polymers via covalent linkages. Improvements in glass transition temperatures and ignition resistance are realized when the resultant phosphorus-containing polymers are compared to conventional bisphenol-A based epoxy thermoplastics. The phenol-functionalized phosphine oxide species of the present invention may be useful as replacements for tetrabromobisphenol-A in FR-4 laminates applications.
It is an objective of the present invention to provide a cross-linking agent which yields superior performance when compared to the previously known commercially available non-halogenated materials.
Still another aspect of the present invention is directed to a compound which includes the glycidyl ether derivatives of Formula I, shown as the following chemical structural Formula V: 
wherein R is as defined above.
Yet another aspect of the present invention is directed to epoxy vinyl ester resins produced by reacting the compound of Formula V above with carboxylic acids which contain an unsaturated functionality to produce the compounds as shown in the following chemical structure of Formula VIII: 
wherein R is defined above and R1 is a moiety containing an unsaturated functionality such as a moiety having the following formula: 
where Rxe2x80x3 is a hydrogen or an alkyl group from C1-C20 and Rxe2x80x2xe2x80x3 may be a hydrogen, an alkyl group from C1-C20, or a carboxylic acid group or ester derivatives thereof. For example Rxe2x80x2 may be a methacrylate, acrylate, maleate, fumarate and the like.